Qgen support for instr()

IMPALA-3973 adds support for the instr() builtin to have a start position and #occurances with instr(), ie. allows searching forwards and backwards (position < 0) some number of matches.

Oracle and some other databases support this: http://docs.oracle.com/database/121/SQLRF/functions089.htm#SQLRF00651

However, the code is pretty tricky and we should get support in the query generator to test this fn against other databases (Oracle in this case).